The invention herein described relates generally to the extrusion of composite profile strips and, more particularly, to the extrusion of a multilayer tread strip for use in the manufacture of a vehicle tire.
Composite profile strips heretofore have been extruded by extrusion apparatus wherein two or more extruders disposed adjacent one another feed respective components (such as rubber or plastic compounds) along respective flow channels in an extrusion head to a die. Flow channels in the head lead to a shaping insert and thereafter to a die wherein the different components are combined for discharge through a common die outlet to form a composite profile strip.
When extruding compounds such as rubber, the rubber is compressed as it is conveyed to the die and then it swells upon exiting the die to atmospheric pressure. This phenomena is referred to as die swell. In a typical system for extruding a composite profile tread strip for use in the manufacture of a vehicle tire, the extruders typically operate at differing throughput capacities and at different pressures for conveying through the extrusion head the rubber compounds which typically have different viscosities. As a result, the rubber compounds may swell at different rates. Also, a higher pressure component may overpower, i.e., swell into, a lower pressure component as they pass through and out of the extrusion die.
In the past, the shaping inserts and dies were designed to compensate for the die swell problem. This was a time consuming and complex procedure. Moreover, the die design would be optimized for a single extrusion rate at which the extrusion apparatus was intended to operate. Accordingly, these prior extrusion systems did not lend themselves to variable extrusion rates as may be desired in various applications.